Power metal oxide semiconductor field effect transistors (MOSFETs) are commonly used power devices due to their low gate drive power, fast switching speed and superior paralleling capability. Most power MOSFETs feature a vertical structure with source and drain regions on opposite sides of a gate trench filled with polysilicon as gate electrodes. In such structures, the MOS channels are formed along the vertical walls of the trenches. One configuration of a trench MOSFET device includes a gate trench lined with thicker oxide in the lower part of the trench and thinner oxide in its upper part. Thus, the oxide has a stepped structure, there being a step in its thickness. The stepped gate structure with a thicker oxide at the bottom portion of the trench increases the breakdown voltage of the device.
In recent years, split-gate trench structures are developed and preferred for certain applications over conventional trench MOSFETs. A split-gate trench power MOSFET comprises two electrodes in a gate trench. A first electrode serves as the gate electrode to control the channel formation of the MOSFET, and a second electrode serves as shield electrode to decrease the capacitance Cgd between drain electrode and gate electrode. Generally, the gate electrode and the shield electrode are formed in a self-aligned process that uses a single mask to form a set of trenches that are used for both the gate electrode and the shield electrode. Since the shield electrode is at source potential, it usually extends to surface and is further coupled to a pick-up structure at a terminal region which is located at sides of the MOSFET. This increases the source electrode resistance. Furthermore, extra masks are needed to create such connection and thereby increasing the cost of manufacturing.
It is within this context that embodiments of the present invention arise.